Role of BkdR, a Transcriptional Activator of the SigL-Dependent Isoleucine and Valine Degradation Pathway in Bacillus subtilis

Abstract

ABSTRACT A new gene, bkdR (formerly called yqiR ), encoding a regulator with a central (catalytic) domain was found in Bacillus subtilis . This gene controls the utilization of isoleucine and valine as sole nitrogen sources. Seven genes, previously called yqiS , yqiT , yqiU , yqiV , bfmBAA , bfmBAB , and bfmBB and now referred to as ptb , bcd , buk , lpd , bkdA1 , bkdA2 , and bkdB , are located downstream from the bkdR gene in B. subtilis . The products of these genes are similar to phosphate butyryl coenzyme A transferase, leucine dehydrogenase, butyrate kinase, and four components of the branched-chain keto acid dehydrogenase complex: E3 (dihydrolipoamide dehydrogenase), E1α (dehydrogenase), E1β (decarboxylase), and E2 (dihydrolipoamide acyltransferase). Isoleucine and valine utilization was abolished in bcd and bkdR null mutants of B. subtilis . The seven genes appear to be organized as an operon, bkd , transcribed from a −12, −24 promoter. The expression of the bkd operon was induced by the presence of isoleucine or valine in the growth medium and depended upon the presence of the sigma factor SigL, a member of the sigma 54 family. Transcription of this operon was abolished in strains containing a null mutation in the regulatory gene bkdR . Deletion analysis showed that upstream activating sequences are involved in the expression of the bkd operon and are probably the target of bkdR . Transcription of the bkd operon is also negatively controlled by CodY, a global regulator of gene expression in response to nutritional conditions.